1. Field of the Invention
This invention relates to new "drywall" compositions and methods for making the same that are useful in the manufacture of wallboard for covering walls and ceilings in construction applications. More particularly, this invention is directed to a novel wallboard composition comprising a unique combination of synthetic binders selected for their ability to establish a strengthened permanent bond in the final dry state, in combination with an expanded mineral such as Perlite which largely reduces the amount of gypsum present in the wallboard product from what has been required by previous gypsum wallboard formulations. This reduction in the amount of gypsum present in the wallboard formulation in turn reduces the weight of the wallboard structure while maintaining its strength. Moreover, the synthetic binders uniquely cross-link with the expanded mineral to form a much stronger bond between the constituent components of the wallboard core material than that which has been available in previously utilized or known wallboard products. In a preferred embodiment, the lightweight, strengthened wallboard of the present invention also comprises a covering veneer that is applied to the top ply of the face paper to provide increased strength, moisture resistance, and fire retardency, and the back paper top ply is treated to provide increased flexural strength. Additionally, this invention relates to the unique manufacturing process to produce the wallboard composition of the present invention in order to create a lightweight, strengthened, moisture resistant, and fire retardant wallboard used to cover walls and ceilings in construction applications. Still further, this invention relates to the apparatus for manufacturing the wallboard composition of the present invention, including a method and apparatus for economically converting a standard gypsum wallboard manufacturing facility into a facility for manufacturing wallboard of the present invention.
2. Description of the Background
Conventional gypsum drywall has been utilized for approximately the past fifty years in the construction industry with gypsum comprising the primary core ingredient. The manufacture of gypsum drywall is presently an expensive, complex, difficult, and tightly controlled manufacturing process. The gypsum wallboard manufacturing process today entails several elaborate steps with significant environmental concerns, both internally and externally, regarding the product itself and the manufacture thereof. An increasingly shortened supply of domestic gypsum rock remains available today, which has necessitated the development and usage of synthetic gypsum as a substitute. However, the production of synthetic gypsum requires an extremely complex synthetic gypsum production facility. Such facilities include FGD (flu gas desulferization) gypsum production plants which are required by the nature of the manufacturing process to be located next to power plant facilities. These power plants utilize high sulfur coal, which is predominate in the Eastern United States, to generate power. The waste produced by these power plants is classified and desulferized into synthetic gypsum. This synthetic gypsum is then calcined and used as a substitute for natural gypsum for use in the wallboard manufacturing process. Given the significant risk of detrimental long-term health effects of a waste slag and coal product, the processing and use of such synthetic gypsum has also fueled environmental concerns. It is an object of the present invention to provide a new and distinctly different environmentally safe class of wallboard for use in the construction industry that utilizes environmentally friendly synthetic adhesives.
The continuously depleting supply of gypsum coupled with the rising demand for wallboard products has caused the price of gypsum and gypsum-based products to rise substantially over recent years. In the field of gypsum wallboard composition, relatively low prices of materials have kept the core of gypsum wallboard unchanged for the better part of the 20th century. However, given the booming construction industry and the increasing demand for housing, the demand for wallboard products has significantly exceeded the available manufactured supply of wallboard. This increased demand has dramatically driven the costs of wallboard products upward. Likewise, the need to supplement the natural gypsum wallboard products with the more costly synthetic gypsum products have also driven up the costs of wallboard products. These increasing cost factors have established a need for a lightweight, strengthened, and re-engineered wallboard product that minimizes the amount of gypsum present in the wallboard formulation.
Attempts have been made in the past to both strengthen and lighten traditional wallboard products, but such efforts have evidenced the addition of substantial costs to the finished product. For example, attempts have been made in the past to use a very low percentage of an inorganic or synthetic binder in wallboard formulations, typically 1% to 2%, in an effort to slightly effect the strength of the wallboard product. However, the amount of binder required to substantially increase strength and remain cost effective has not been realized. As disclosed herein and as a part of the present invention, it has been found that by placing the equipment needed to polymerize the base components of the synthetic binder on-site at the wallboard manufacturing facility, manufacturing costs may be greatly reduced.
Modem gypsum wallboard manufacturing facilities are very expensive in and of themselves, comprising numerous pieces of complex manufacturing and material handling equipment. Traditionally, the removal of the gypsum rock from gypsum mines or quarries is more difficult than strip or surface mining the softer Perlite ore from the mountain or ranges. After mining, the harder and larger gypsum rocks are crushed and reduced to smaller sizes and conveyed to where these smaller rocks are crushed into tiny particles. Next, the crushed gypsum is processed through a complex Calcining system involving a roller mill, a Calcining kettle, an imp mill and/or GC mill to reduce the gypsum fines into a chalk-like gypsum aqueous slurry. This Calcining system and process is expensive as it involves flash-drying and heating the gypsum land plaster or gypsum slurry in order to remove much of the water from the material. Following this dehydration process, the gypsum stucco is stored in holding bins and fed into equipment such as a pin mixer and a screw type conveyer. Water is again added along with other ingredients such as foams, starches, cementious materials and other chemicals to form the final prepared gypsum slurry. The gypsum paste is then spread onto and compressed between facing and backing paper and is cut further down the line. Next, a complex high temperature kiln dries the green gypsum board for approximately one hour or more, which is begun at lower temperatures (approximately 250.degree. F.), then to a higher temperature (approximately 600.degree. F.), and down again to exit from the kiln at lower temperatures (approximately 200.degree. F.), leaving the gypsum board virtually moisture-free. This complex system of processing and material handling equipment is extremely expensive, such that the start-up of a new facility to manufacture a new type of wallboard has in the past been cost prohibitive. It would therefore be advantageous to provide a means by which an existing manufacturing facility could be modified at little expense to produce a strengthened and lighter weight wallboard product.
Perlite and other minerals have previously been used in wallboard construction as a filler, and has likewise been used in a variety of other industrial uses, including abrasives, acoustical plaster and tile, charcoal barbecue base, cleanser base, concrete construction aggregates, filter aid, fertilizer extender, foundry ladle covering and sand additive, inert carrier, insulation board filler, loose-fill insulation, molding filler medium, packaging medium, paint texturizer, propagating cuttings for plants, refractory products, soil conditioner, tile mortar aggregate, and lightweight insulating concrete for roof-decks. Perlite is a glassy volcanic rock having the unusual characteristic of expanding to about 20 times its original volume when heated to an appropriate temperature within its softening range. The resultant expanded product finds a variety of industrial and constructional applications owing to the material's low density with attendant properties of low thermal conductivity and high sound absorption.
Petrographically, Perlite can be described as a glassy, volcanic, rhyolitic rock having a pearl-like luster and usually exhibiting numerous concentric cracks resembling an onionskin in appearance. Chemically, crude Perlite is essentially a metastable amorphous aluminum silicate. A typical average chemical analysis of Perlite would show a range of 71% to 75% SiO.sub.2, 12.5% to 18.0% Al.sub.2 O.sub.3, 4 to 5 percent K.sub.2 O, 1% to 4% sodium and calcium oxides, and minor amounts (traces) of metal oxides. Perlite is chemically inert and has a pH of approximately 7. The specific gravity of Perlite ranges from 2.2 to 2.4 (139 to 150 pounds per cubic foot) and has hardness between 5.5 and 7 (Mohs' scale). Crude Perlite may range from transparent light gray to glassy black in color; however, the color of Perlite when expanded will range from snowy white to grayish white.
Commercially, the term "Perlite" also includes the expanded product. When particles of Perlite are heated to a soft consistency, the combined water present (2% to 5%) in the glass vaporizes, forming steam that expands each particle into a mass of glass foam. The original volume of the crude Perlite may be expanded 4 to 20 times at temperatures between 1,200.degree. F. and 2,000.degree. F. Expanded Perlite may be a fluffy highly porous material or may be composed of glazed glassy particles having a low porosity. Dependent upon the inherent physical properties and processing variables, the bulk weight of expanded Perlite usually ranges from 2 to 20 pounds per cubic foot.
Specifications have been established by the American Society for Testing and Materials (ASTM) for the sizing and bulk density of expanded Perlite aggregate used for plaster and insulating concrete. Perlite for filter media uses and for many other uses usually follows specifications for proper sizing and other properties recommended by producers.
Perlite (expanded) can be graded by density in pounds per cubic foot, and classified by product number or trade name for producer and user identification. The expanded product can weigh as little as 2 pounds per cubic foot, but the most widely used bulk-density grade range is from 7 to 15 pounds per cubic foot. The range of expanded Perlite utilized in the wallboard composite core of the present invention is 3 to 10 pounds per cubic foot, but preferably Perlite weighing 4 pounds per cubic foot is utilized in the formulation of the present invention. It has been found that the more friable cryogenic grades in the 3 to 4 pound range are favorable over the heavier grades ranging from 5 to 10 pounds per cubic foot. Grades typical to this range include concrete, plaster, and cavity fill or masonry. The particle size ranges from 100 to 2,000 microns, and preferably from 200 to 1000 microns. Preferably, the expanded Perlite will have a particle size ranging from no larger than 10 mesh sieve size and no smaller than 200 mesh sieve size measured on standard screen scale. The particle size of the preferred expanded Perlite is directly related to the strength of the wallboard core in the aspect of fusion. Particles that are too large tend to space gypsum crystal growth too far apart, and particles that are too small don't allow enough area for the gypsum crystal to fuse onto.
The expanded product is bagged for shipment and generally will be in volume of 4 cubic feet per bag. The expanded product is generally shipped via truck or rail. If by rail, the expanded product may be shipped in bulk dry density designed transport cars.
Expanded Perlite, depending on the expansion process and the grade of Perlite, can affect the expanded weight and can be used in the production of many products where weight is an important factor. In the construction industry, Perlite's incombustibility and low water absorption make it a superior insulating material. Perlite plaster aggregate is used extensively to fireproof structural steel construction and to reduce the weight of interior walls and ceilings. Perlite concrete aggregate roof-decks also insulate and save weight. Expanded Perlite is an important component of roof insulation (gypsum) board, masonry (cavity fill), and floor and wall tiles.
Some of the many important applications of Perlite include its use as an insulator (in cryogenic technology) to hold solidified gases such as liquid oxygen at extremely low temperatures, to absorb oil spills on water and wet surfaces, to clean up effluents containing oily wastes, and as an additive in molding sands.
In sum, while perlite has found a variety of uses in the construction industry, and even as a filler in wallboard products, it has not heretofore been effectively employed as a catalyst for significantly reducing the amount of gypsum required in the wallboard formulation.
Further, the green and/or gray-colored facing and backing paper used on standard gypsum wallboard is commonly low-grade and recycled, and performs poorly under rainy or wet surface conditions during shipping, construction, and the installation process. Weight factors of the gypsum drywall/sheetrock, as commonly termed, have been an ongoing concern during transportation and installation, as have general safety issues, particularly in hanging ceiling boards. Breakage and loss of material is an adverse factor during brittle gypsum board installation. It would therefore also be advantageous to provide an improved facing and backing paper lacking the shortcomings evident in the prior art.